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Old 04-06-2021, 04:01 PM   #1
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Default 5.0 runs rough and stalls at low idle

My 5.0 runs great through the midrange and up top, but at idle it will slowly lose rpm, run rough and eventually stall. If I give it some throttle, it perks back up and will continue to run. Once back at idle, it does the same... It starting this last season, but seems to be worse now. I have changed plugs, cap, rotor, rebuilt carb last season( 8 months )I am planning on another carb clean, but any suggestions would be great.
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Old 04-06-2021, 04:55 PM   #2
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Have you replaced the fuel/water separating filter?
My Mercarb s were very particular about water in the fuel.
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Old 04-07-2021, 05:22 PM   #3
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I had the same issues. I ended up buying a rebuilt carb from Guaranteed Carbs around $300 exchange. It runs perfect now, I only had to adjust the choke. There are some tiny ball valves I could not get out to clean and do not come in a repair kit.
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Old 04-09-2021, 04:34 AM   #4
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I have changed the filter, and I am going to have the carb rebuilt soon. I ran it today, and when I look down through the top of the carb at idle, I have fuel dripping into both chambers, that is normal... right?
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Old 04-09-2021, 10:13 AM   #5
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Dripping fuel is not normal.
Sounds like your idle issue is related to an over rich condition as a result of the dripping.

This can happen if the float level is to high or needle/seat are dirty. Another possibility
is the float is saturated or if a brass float, it has a pinhole and is filling with fuel and not rising to the point of closing the needle fully.

I had this same issue and found it was a combination of a bad float coupled with a dirty needle. I rebuilt both carbs with kits from Mikes Carburetor and problem solved.
You will have a choice of a solid shaft needle or a spring loaded one. There was a bulletin on this and the float adjustment is different for each.

John
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Old Yesterday, 08:51 PM   #6
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I removed the carb, cleaned again, and actually adjusted the float to 22mm from 18... fuel still drips at idle. The boat runs excellent at WOT, but blubbers and eventually dies at idle, I would agree on the rich condition. Last season I replaced a bunch of things at once, one being the fuel pump. I replaced the stock mercruiser with a low pressure fuel pump from " the fuel pump factory" I am now wondering if it is putting out too much fuel, floating the float and dripping into the Venturi..... I haven't checked the fuel pressure due to the rigid line... that being a giant PITA. Do you think those pumps are junk?
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